1. Field of the Invention
The present invention relates to a liquid crystal display device. More specifically, the present invention relates to an electrode shape of a fringe field switching (FFS) type liquid crystal display device.
2. Description of Related Art
In recent years, in substitution for a conventional cathode-ray tube, new display devices having a thin flat-type display panel using principles of liquid crystals, electroluminescence or the like have been used a lot. A liquid crystal display device which represents these new display devices has characteristics in that it can be driven with a low power voltage, in addition to its thinness and lightness. The liquid crystal display device includes a liquid crystal layer disposed between two substrates. One substrate is an array substrate forming a display area in which a plurality of pixels are arranged in matrix, and the other substrate is a color filter substrate.
In particular, in a thin film transistor (TFT)-type liquid crystal display device, a TFT which is a switching element is provided in each pixel on the array substrate, and each pixel is able to independently carry a voltage to drive a liquid crystal layer, thereby making it possible to achieve the display of high quality with less crosstalk. Each pixel includes a scan line (gate line) that controls ON/OFF of the TFT, and a signal line (source line) for inputting image data. Each pixel typically corresponds to an area surrounded by the scan line and the signal line.
An In-Plane switching (IPS)-type liquid crystal display device has one array substrate on which a plurality of pixel electrodes and opposed electrodes (common electrodes) are alternately arranged, so as to apply a substantially-horizontal electric field to the substrate surface for display. The IPS type liquid crystal display device has better view angle property compared with a typical TN (Twisted Nematic) type. However, the conventional IPS-type liquid crystal display device has a smaller light transmittance compared with the typical TN type.
As a system in which this defect is improved, a fringe field switching (FFS) system has been suggested (for example, Japanese Unexamined Patent Application Publication Nos. 11-202356, 2008-197343). The FFS-type liquid crystal display device is the system of achieving the display by applying a fringe electric field (an oblique electric field including both components of a horizontal electric field and a vertical electric field) to a liquid crystal layer. In the FFS-type liquid crystal display device, the pixel electrode and the opposed electrode are formed on one array substrate as is similar to the IPS system. However, the pixel electrode and the opposed electrode are overlapped with each other with an insulation film interposed therebetween. Typically, the lower electrode has a plate-like shape (the lower electrode may be a plurality of brunch-like electrodes). Furthermore the upper electrode includes a plurality of branch electrode parts electrically connected in common, and gap parts each of which being interposed between the branch electrode parts.
The pixel electrode may be formed either in the lower electrode or the upper electrode. In the transmission type, the pixel electrode and the opposed electrode are both formed of transparent conductive films. In the reflective type, the upper electrode may be formed of a transparent conductive film and the lower electrode may be formed of a conductive film having high reflectance rate. The liquid crystal layer is driven by the fringe electric field between the upper electrode and the lower electrode in the FFS system, thereby making it possible to drive the liquid crystal layer on the branch electrode parts and gap parts of the upper electrode. For example, in the transmission type, the pixel electrode and the opposed electrode are formed of transparent conductive films. Thus, the FFS system achieves improved light transmittance than the IPS system that transmits little light on the pixel electrode and the opposed electrode.